For most low-side switches I've seen, an NMOS device is used. I am wondering, is it not possible to use a PMOS device in a low-side switch?
Yes, it's possible. But just like with a high-side NMOS switch, you'd need to drive the gate beyond the supply rail (below Gnd, in this case) in order to switch it fully on.
While there's some motivation to use high-side NMOS switches — they perform better than the equivalent PMOS devices — there's no equivalent motivation to use a PMOS on the low side.